CEPHALOPODA. ie 
Nanno aulema,] 
tions through which the animal and its shell pass from their primitive condition 
onward, and we take the liberty of quoting this passage: “The visceral sac of the 
animal had obtained a considerable size. Its form was pointed posteriorly. The 
mantle had secreted a shell of like form. This shell was thus quite open and of 
conical shape. It now formed but a single chamber which was both initial and 
habitation chamber, wholly filled by the animal. With the growth of the animal 
the shell was, naturally, lengthened on the anterior margin. As the animal [shell] 
at last became too heavy and had need of a hydrostatic apparatus in order to raise 
and sink itself, the air-chambers, by which the hydrostatic problem is solved for the 
tetrabranchiates, were formed. The first of these originated in this way: On one 
side of the upper portion of the visceral sac a circular and almost enclosed constric- 
tion was produced. The fold of the mantle thus formed deposited shell-matter 
making an inclined wall and a division of a part of the originally open initial 
chamber. The resulting chamber was empty and formed the first air-chamber. 
This chamber is, thus, bounded by only one septum and in this case lies behind the 
wall corresponding to the first septum in Nautilus. It therefore corresponds to the 
initial chamber in that genus. As it here has the same function as the other air- 
chambers, I have termed it the first air-chamber, although, in fact, it is a remnant 
of the open initial chamber. Moreover, the second air-chamber is probably formed 
in part from the anterior portion of the initial chamber. The visceral sac of the 
animal was now divided by a constriction into an anterior and posterior portion. 
* * * # The anterior portion now forms the actual habitation chamber, but the 
great visceral sac also fills the posterior portion. The growth of the shell progresses; 
the shell is again lengthened about the aperture. The animal becomes again too 
heavy and must form another air-chamber. It loosens itself from the wall of the 
conch, the visceral sac extends itself along the constriction and the animal moves 
forward a step in its shell. The mantle surfaces form a new septum and about the 
elongated portion of the visceral sac the calcareous deposit takes the form of a 
sheath or siphonal funnel. Thus originates the siphonal cord of the animal. 
Hence the sipho of Endoceras belemnitiforme must have had its origin in a difterenti- 
ation of the visceral sac.” (Op. cit., pp. 6, 7.) 
The praseptal cone of Nanno must be regarded as a great protoconch expressing 
in its form the primitive nature of the longicones and brevicones of the tetra- 
branchiates. 
It may be suggested that the solidification of the preeseptal cone may have been 
to some extent due to such secondary causes as have produced the solid guard in 
Belemnites. The appearance of these siphones and the crystalline structure of their 
